Earlier we have identified a pedigreed breeding colony of feral mice (CZII) which lack MMTV proviral DNA in their germ line. We have begun to assess the role of genetically transmitted MMTV proviral genomes in mammary neoplasia by genetically introducing individual MMTV proviral genomes from Balb/c and C3H/Hef mice into the CZII genetic background. These mice are currently being bred and monitored for the development of mammary tumors. To facilitate this program we have identified, in collaboration with Dr. C. Kozak, the chromosomal locations of the Balb/c proviral genomes using mouse-hamster somatic hybrids. The Balb/c proviral genomes are linked respectively to chromosomes 12, 16, and 18. The MMTV related sequences on chromosomes 12 and 18 represent complete proviral genomes where as the sequences on chromosome 16 correspond only to the MMTV LTR and leader sequences. The expression of the Balb/c MMTV proviral genes was determined in normal tissues and various tumors of Balb/c mice. Low levels of MMTV genomic and env mRNA was detected in several normal tissues. High levels of a novel 1.6 and 1.2 kbp MMTV RNA was detected respectively in lactating mammary gland and salivary gland tissue. Each of these RNA species hybridized only with LTR portion of the viral genome. High levels of the 1.6 kbp RNA were also detected in all pristine induced Balb/c plasma cell tumors. Some of these tumors, as well as carcinogen induced mammary hyperplastic alveolar nodule (HAN) and HAN derived mammary tumors also expressed high levels of genomic and env MMTV RNA.